Compressed air supply for the operation of moving bed filters

ABSTRACT

A method for supplying compressed air for the operation of moving bed filters by way a compressor is characterized by controlling the pressure of compressed air from the compressor by controlling the rotational speed of the compressor. A system for supplying compressed air is also provided. By use of the described method and system, substantial energy savings of approximately 50 percent can be achieved.

TECHNICAL FIELD

The present invention relates generally to a method and a system for the supply of compressed air for operating granular bed filters and/or reactors of the moving bed type and particularly for operating air lift pumps in such filters.

BACKGROUND ART

Moving bed filters are commonly used for treatment of water and waste-water. The granular media filter bed (hereafter filter bed) of such moving bed filters is kept in a slow motion by a transport device transporting granular filter media from the lower part of the filter bed to the top of the filter bed often via a washer for granular filter media, so-called media washer, in which the granular filter media is washed to remove pollutants that have been separated in the moving bed filter.

Examples of moving bed filters are found in the U.S. Pat. No. 6,426,005 B 1, U.S. Pat. No. 4,126,546, U.S. Pat. No. 4,197,201, and U.S. Pat. No. 4,246,102, which are incorporated herein by reference.

One or more air lift pumps are normally used as such transport device. The air lift pump is operated with compressed air. In principle an air lift pump is a tube immersed in a liquid, which may contain particles, with the inlet part of the tube at a lower level than outlet part thereof and in which compressed air is introduced close to the inlet part. Since the air/liquid/particle mixture in the air lift pump will have a lower density than the surrounding liquid or liquid particle mixture a pumping action will result.

Air lift pumps offer a simple means of transporting liquids containing particles and the operating cost will largely depend on the cost of said compressed air. This cost is dependent on air pressure and air flow e.g. volume per unit of time. Air pressure is the factor that has the greatest influence on the cost of operating an air lift pump.

Traditionally compressors giving an air pressure of typically 7 bars and blowers operating at an air pressure of approx. 1 bar have been available. Since the pressure from a blower has not been sufficient for operating air lift pumps in moving bed filters the standard means for such operation has been the traditional compressor. The pressure of the compressed air exiting from such a compressor is too high for direct use for operating an air lift pump in a moving bed filter and the compressed air has therefore been subjected to a pressure reduction before being used in the air lift pump. The practice of first creating a high pressure and then reducing the pressure before using the compressed air leads to unwanted energy losses.

The reason for this practice is that no suitable compressors offering a pressure between the too high pressure of the traditional compressor and the too low pressure of the blower have been available.

This has been changed with the advent of the mid pressure compressor (for instance claw compressor), which can produce compressed air of for instance 2.2 bars, i.e. lower than the conventional compressor but higher than the blower. The pressure of the compressed air delivered by a mid pressure compressor would appear suitable for operation of moving bed filters.

However, surprisingly enough replacing the conventional compressor with such a mid pressure compressor for the operation of moving bed filters leads to problems. For instance in the moving bed filter system using a conventional compressor, the compressor is normally controlled in such a way that it pumps compressed air into a holding tank for air which feeds air into the filter system. The compressor is operated in on/off mode in such a way that a high pressure level and a low pressure level are set and when the air pressure in the tank has reached the high pressure the compressor is turned off and when the air pressure in the tank reaches the low pressure the compressor starts again. If a mid pressure compressor is used instead of the conventional compressor the system will fail. The pressure of the mid pressure compressor is close to the pressure needed by the air lift pump and therefore it is not possible to use it in the same system as the conventional compressor, since the on/off operation to control the compressor will lead either to too frequent starts and stops that will damage the compressor or the lower pressure set for the start of the compressor will be too low for operating the air lift pump.

The use of a mid pressure compressor will also lead to a system that is highly sensitive to condensation of water in the tubes for the compressed air. The condensed water together with the low pressure will render the system inoperable.

SUMMARY OF INVENTION

An object of the present invention is to provide a method and a system of the kind initially mentioned, wherein the drawbacks of prior art are eliminated or at least minimized.

According to a first aspect of the invention, there is provided a method as defined by the features of the characterizing portion of appended claim 1.

According to a second aspect of the invention, there is provided a system as defined by the features of the characterizing portion of appended claim 7.

By use of the described method and system, substantial energy savings of approximately 50 percent can be achieved.

An electrical frequency control device is preferably used for the control of the rotational speed of an electrical motor.

The rotational speed of a combustion engine is preferably controlled by controlling the fuel/air mixture fed to the engine.

In a preferred embodiment, the moving bed filter comprises an air lift pump.

In a preferred embodiment, a controller and a pressure sensor adapted to sense the pressure in the conduit are provided, wherein the controller is adapted to control the speed of the compressor in dependence of the sensed pressure in the conduit.

The compressor is preferably a claw compressor.

In a preferred embodiment, the compressor is devoid of lubrication in the air part. Thereby, compressed air free from traces of lubricants can be produced for use in sensitive applications, e.g. drinking water production.

In a preferred embodiment, a heat exchanger is adapted to cool compressed air in the conduit in order to condense the water vapor. Thereby, the system is less sensitive to condensation of water in the tubes for the compressed air.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 illustrates a conventional system according to prior art.

FIG. 2 illustrates a system according to the invention.

DESCRIPTION OF EMBODIMENTS

In the following, a detailed description of a method and a system for supply of compressed air will be given.

FIG. 1 shows a conventional system according to prior art with a compressor 1 which will normally be of the piston or screw compressor type. The compressed air in the compressed air line 2 passes a lubricant removal filter 3 for the removal of drops of lubricant which may have been entrained in the compressed air in the compressor. An air holding tank 4 for the compressed air is included in the system and the air pressure is controlled by an on/off control system with a pressure sensor 5 connected to a control box 6 which in turn gives a signal that controls the motor of the compressor in such a way that the compressor is stopped when the air pressure in the holding tank reaches a certain level and starts the motor of the compressor when the air pressure in the holding tank has fallen below a certain predetermined value. The pressure of the compressed air is finally reduced to the operating pressure needed for the transport device(s) of the moving bed filter(s) connected to the system in a pressure reducing device 7, wherein the pressure of the compressed air is reduced from approximately 7 Bars to a pressure of approximately 3-4 Bars and a further pressure reduction takes place in connection with the flow control devices in a pneumatic cabinet 8 used for the subsequent distribution of the compressed air to the transporting device(s) of the moving bed filter(s).

FIG. 2 shows the proposed new system, generally designated 10, with a mid pressure compressor 11 with a motor with its rotational speed controlled by a controller in the form of a control box 14 which receives a signal from a pressure sensor 13, which senses the pressure in a conduit in the form of a compressed air line 12. The controller keeps the pressure in the compressed air line at close to a predetermined pressure in the range of 1.2-2.0 Bars, preferably approximately 1.5 Bars. The compressed air is optionally cooled in a heat exchanger 15, for instance with water provided through pipe 16. The compressed air line is arranged with a slope at least between the heat exchanger and a pneumatic cabinet 18 towards a lowest point where a valve 19 is used for evacuating condensate from the heat exchanger and the compressed air line. The heat exchanger 15 is arranged so that any condensate formed will be evacuated through the valve 19. Likewise the compressed air line will be arranged so that any condensate formed will flow to the lowest point of the line and be evacuated through valve 19.

The compressed air needed for the transport device(s) of the moving bed filter(s) connected to the system is distributed to such transport device(s) via flow control devices in the pneumatic cabinet 18 wherein also the pressure of the compressed air is finally adjusted.

A preferred embodiment of a method and a system for supplying compressed air for the operation of moving bed filters according to the invention has been described. It will be realized that this embodiment can be modified without departing from the inventive idea as defined in the appended claims. 

1. A method for supplying compressed air for the operation of moving bed filters by means of a compressor, characterized by controlling the pressure of compressed air from the compressor by controlling the rotational speed of the compressor.
 2. The method according to claim 1, comprising using an electrical frequency control device for the control of the rotational speed of an electrical motor.
 3. The method according to claim 1, comprising controlling the rotational speed of a combustion engine by controlling the fuel/air mixture fed to the engine.
 4. The method according to claim 1, comprising condensing water vapor in the compressed air in an air supply system downstream of the compressor and discharging the condensed water.
 5. The method according to claim 1, comprising cooling the compressed air in order to condense the water vapor.
 6. The method according to claim 1, wherein a claw compressor is used.
 7. The method according to claim 1 any of claims 1 6, wherein the pressure of compressed air from the compressor is kept in the range of 1.2-2.0 Bars.
 8. A system for supplying compressed air for the operation of a moving bed filter, the system comprising: a compressor (11), a conduit (12) interconnecting the compressor and the moving bed filter, characterized in that the compressor is a variable speed compressor.
 9. The system according to claim 8, wherein the moving bed filter comprises an air lift pump.
 10. The system according to claim 8, comprising a controller (14) and a pressure sensor (13) adapted to sense the pressure in the conduit (12), wherein the controller is adapted to control the speed of the compressor in dependence of the sensed pressure in the conduit.
 11. The system according to claim 8, wherein the compressor is a claw compressor.
 12. The system according to claim 8, wherein the compressor is devoid of lubrication in the air part.
 13. The system according to claim 8, comprising a heat exchanger (15) adapted to cool compressed air in the conduit (12).
 14. The system according to claim 8, wherein the compressor is adapted to keep the pressure in the conduit at close to a predetermined pressure in the range of 1.2-2.0 Bars.
 15. The system according to claim 9, comprising a controller (14) and a pressure sensor (13) adapted to sense the pressure in the conduit (12), wherein the controller is adapted to control the speed of the compressor in dependence of the sensed pressure in the conduit.
 16. The method according to claim 2, comprising condensing water vapor in the compressed air in an air supply system downstream of the compressor and discharging the condensed water.
 17. The method according to claim 3, comprising condensing water vapor in the compressed air in an air supply system downstream of the compressor and discharging the condensed water. 